Various rubber compositions for components for various products, such as for example tires, contain particulate reinforcement comprised of a combination of precipitated silica with a silica coupling agent for the silica as well as rubber reinforcing carbon black. Coupling agents are used for coupling the precipitated silica to diene-based elastomers contained in the rubber composition.
Representative of such coupling agents are, for example, bis(3-trialkoxysilylalkyl) polysulfides which contain an average of from about 2 to about 4 connecting sulfur atoms in their polysulfidic bridge, such as for example those comprised of bis(3-triethoxysilylpropyl) polysulfide, and alkoxyorganomercaptosilanes.
However, use of such polysulfide based coupling agent, particularly such coupling agent which contains an average of from about 3 to about 4 connecting sulfur atoms in its polysulfidic bridge, can tend to liberate free, or otherwise make available, sulfur and to thereby promote a degree of premature crosslinking of the rubber with an attendant increase in its viscosity during elevated temperature mixing of the uncured rubber. Such phenomenon is well known to those having skill in such art.
It is desired herein to utilize a bis(3-trialkoxysilylalkyl)polysulfide, or alkoxyorganomercaptosilane, as a coupling agent for precipitated silica in a diene-based elastomer composition in a manner which provides a lower rubber viscosity for the resultant rubber composition product obtained from the internal rubber mixer.
In practice, the diene-based elastomer compositions typically contain a processing aid in a form of a fatty acid salt. Such fatty acid salt may be, for example, a fatty acid salt formed in situ within the rubber composition as a reaction product of zinc oxide and at least one long chain fatty carboxylic acid such as, for example, stearic acid, palmitic acid and oleic acid and particularly fatty acid comprised of a mixture of stearic acid, palmitic acid and oleic acid.
Alternatively, it is envisioned that the fatty acid salt may be introduced into the rubber composition as a pre-formed fatty acid salt as a composite of a fatty acid anion and cation such as, for example, zinc, calcium, magnesium and aluminum. For example, such fatty acid salt may be a calcium fatty acid salt or zinc fatty acid salt.
Historically, one method of providing a lower viscosity rubber composition from an internal rubber mixer is to simply significantly increase the aforesaid fatty acid salt processing aid content of the rubber composition.
However, the inclusion of a significantly increased fatty acid salt content in the rubber composition is envisioned herein as tending to dilute, or otherwise degrade, various physical properties of the rubber composition, particularly various physical properties of the eventually sulfur cured rubber composition.
While the mechanism might not be fully understood, it is envisioned herein that the fatty acid salt tends to be absorbed onto the porous precipitated silica surface to thereby inhibit some of the coupling agent promoted coupling taking place between the silica and diene-based elastomer.
For this invention it has been observed, and is considered herein a discovery that, by delayed and divided inclusion of a fatty acid salt, whether as a pre-formed fatty acid salt addition or as a fatty acid salt formed in situ within the rubber composition, which contains precipitated silica reinforcement and coupling agent, during the mixing process, the final viscosity (Mooney viscosity) of the mixed rubber composition can be reduced to create a rubber composition with various of its physical properties improved.
For this invention, it has been found and observed that the delayed fatty acid salt inclusion may be different from an initial and divided inclusion of a fatty acid salt in the preparation of the rubber composition, which is a particular benefit for the practice of this invention in producing a beneficial rubber composition.
In practice vulcanized elastomer products are typically prepared by thermomechanically mixing unvulcanized rubber and various compounding ingredients in a step-wise manner to form a compounded rubber followed by shaping and curing the compounded rubber at an elevated temperature to form a vulcanized product.
Initially, the elastomer (rubber) and various compounding ingredients, typically exclusive of sulfur and sulfur vulcanization accelerators, are typically blended in one or more what are generally referred to as being non-productive thermomechanical mixing stages, or steps, in a suitable internal rubber mixer, in the absence of sulfur and sulfur cure accelerators. Such non-productive mixing is usually conducted at high shear mixing conditions at an elevated temperature in a range, for example, of about 140° C. to 190° C. and often in a range of about 150° C. to 180° C.
Following such non-productive mixing of the rubber composition, sulfur and sulfur vulcanization accelerators (curatives), are mixed with the rubber composition in a final mixing stage, which is often referred to as being a productive mixing stage, or step, also in an internal rubber mixer, to a significantly lower mixing temperature in a range of about 100° C. to about 120° C. to prevent premature curing of the rubber composition.
The rubber composition is typically allowed to cool between the aforesaid various mixing steps, for example, to a temperature below 50° C.
Such non-productive mixing steps and final productive mixing step are well known to those having skill in the rubber mixing art.
For this invention, it is proposed to evaluate a blending of the precipitated silica and diene-based elastomer(s) in at least one preliminary, non-productive mixing step in an internal rubber mixer in the presence of less than 50 percent of fatty acid salt processing aid to minimize initial contact of the precipitated silica with the fatty acid salt processing aid and to thereafter blend the remainder of greater than 50 percent of the fatty acid salt processing aid such processing aid(s) with the silica-containing rubber composition to achieve an enhanced silica-reinforced rubber composition after an initial reaction between the silica and coupling agent.
In such manner, not only would inclusion of a major portion of the fatty acid salt be delayed but the overall inclusion of the fatty acid salt would be divided with only a minor portion of the fatty acid salt being provided together with the silica and silica coupling agent which is believed to be a significant departure from past practice.
Such evaluation is to therefore evaluate what effect would occur if a partial, divided, delayed addition (delayed addition) of greater than 50 percent of the fatty acid salt processing aid would be made after addition of, and therefore after an initial reaction of, the silica and silica coupling agent, to the rubber composition and the resultant silica-containing rubber composition.
Such evaluation is to further evaluate an inclusion of an initial fatty acid salt to the rubber composition coincident with or prior to addition of the precipitated silica and silica with a delayed different fatty acid inclusion to the rubber composition subsequent to the addition of, and in a separate mixing step from, said initial fatty acid salt to the rubber composition in which the later, delayed inclusion of the different fatty acid salt constitutes the major (at greater than 50 percent) portion of the overall fatty acid salt addition to the rubber composition.
The term “phr” as used herein, and according to conventional practice, refers to “parts of a material per 100 parts by weight of rubber, or elastomer”.
In the description of this invention, the terms “rubber” and “elastomer” if used herein, may be used interchangeably, unless otherwise prescribed. The terms such as “rubber composition”, “compounded rubber” and “rubber compound”, if used herein, are used interchangeably to refer to rubber which has been blended or mixed with various ingredients and materials and “rubber compounding” or “compounding” may be used to refer to the mixing of such materials. Such terms are well known to those having skill in the rubber mixing or rubber compounding art.